In 4 color (cyan, magenta, yellow, black) printing, separating the axes of the cyan, magenta and black halftone screens by thirty degrees (15, 45 and 75 degrees) and using a hexagonal halftone screen for the yellow which has axis angles of 0, +60 and -60 degrees, to break up the moire pattern which is produced by the yellow colorant in the prior art method.
In the typical 4-color printing prior art, four square halftone screens are produced which have angles of 0, 15, 45 and 75 degrees. Typically, the three most dominant colorants (cyan, magenta and black) are separated by 30 degrees which is the optimum separation for minimum moire, and the last (yellow, which is least visible) is set to zero degrees. This typically gives an angle of 15 degrees between the yellow and two other colors, usually cyan and magenta. This results in a good print in most cases in that color moire (repetitive patterns in what should be solid colors) caused by inaccurate registration of the various halftone dots in printing is minimized, but in cases where a high quality print is required, a close inspection will show that the yellow colorant set to zero degrees will beat against those 15 degrees away, cyan and magenta, to make an objectionable lower frequency moire. These 2-color moire patterns are then most noticeable in pinks, oranges, tans and greens. Also, the yellow screen is 45 degrees from the black, and this angle also is capable of producing moire. A technique to break up these 2-color moire patterns is needed to increase the quality of 4 color halftone pictures.
The axes of the screens are determined by connecting lines between each dot and its nearest neighbors. The screen angle is the measure of a screen axis with respect to the horizontal.